1. Field of the Invention
This invention relates to antenna arrangements, and in particular to such arrangements comprising a tapered slot antenna and a balun for coupling a feed line with the antenna.
2. Description of Related Art
A tapered slot antenna formed on a substrate is conventionally coupled with a feed line via a balun comprising a straight length of stripline on the main face of the substrate opposite the tapered slot antenna extending at right angles to a slot line extending from the narrower end of the tapered slot. This form of balun has an inherent narrow bandwidth characteristic.
It is an object of the invention to provide an antenna arrangement comprising a tapered slot antenna and a balun for coupling a feed line with the antenna wherein the balun has a broader bandwidth capability than corresponding known arrangements.
According to the invention, an antenna arrangement comprises an antenna, a feed line and a balun for coupling the antenna with the feed line, wherein said antenna comprises a tapered slot in an electrically conductive layer carried on one main face of an electrically insulating substrate, and said balun comprises a non-tapered slot line forming an extension of the narrower end of said tapered slot and terminated by an open-circuit, and a length of stripline carried on an opposite main face of said substrate extending from said feed line and terminated by a short-circuit, said slot line and said stripline each having a 45.degree. twist, the two twists being centred about a common point in the plane of said substrate.
The tapered slot is preferably exponentially tapered.
The section of the stripline between said feed line and said point is preferably aligned with the section of said slot line between said narrower end of the tapered slot and said point.
In a preferred embodiment of the invention, the length of said slot line between said point and said open-circuit is one quarter of the guide wavelength in the slot line at twice the lower operating frequency of said antenna, and the length of said stripline between said point and said short-circuit is one quarter of the guide wavelength in the stripline at twice the lower operating frequency of said antenna.
The open-circuit preferably comprises a circular slot in said conductive layer, the slot having a diameter equal to one quarter of the guide wavelength in said slot line at the upper operating frequency of said antenna.